Nickel-based active material for lithium secondary battery, method of preparing the same, and lithium secondary battery including positive electrode including the nickel-based active material

ABSTRACT

Provided are a nickel-based active material for a lithium secondary battery, a method of preparing the nickel-based active material, and a lithium secondary battery including a positive electrode including the nickel-based active material. The nickel-based active material includes at least one secondary particle that includes at least two primary particle structures, the primary particle structures each including a porous inner portion and an outer portion having a radially arranged structure, and the secondary particle including at least two radial centers.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.15/829,548, filed on Dec. 1, 2017, which claims priority to and thebenefit of Korean Patent Application No. 10-2016-0163897, filed on Dec.2, 2016, in the Korean Intellectual Property Office, the entire contentof which is incorporated herein by reference.

BACKGROUND 1. Field

One or more embodiments of the present disclosure relate to anickel-based active material for a lithium secondary battery, a methodof preparing the same, and a lithium secondary battery including apositive electrode including the nickel-based active material.

2. Description of the Related Art

As portable electronic devices and communication devices develop, thereis a high need for development of a lithium secondary battery having ahigh energy density. However, safety of a lithium secondary batteryhaving a high energy density may deteriorate, and thus improvement inthis regard is needed. As a positive active material of a lithiumsecondary battery, a lithium nickel manganese cobalt composite oxide, alithium cobalt oxide, or the like may be used. However, when such apositive active material is used, migration distances of lithium ionsare determined according to a size of a secondary particle during acharge/discharge process, and efficiency of the charge/discharge processis not high due to the physical distances. Also, a long lifespan of alithium secondary battery is reduced due to cracks generated in primaryparticle units as a charge/discharge process is repeated. In addition,battery resistance increases and battery capacity characteristics do notreach a suitable and/or satisfactory level, and thus, there is a need toimprove the positive active material.

SUMMARY

One or more embodiments include a nickel-based active material for alithium secondary battery, the nickel-based active material havingimproved lithium ion availability.

One or more embodiments include a method of preparing the nickel-basedactive material.

One or more embodiments include a lithium secondary battery including apositive electrode including the nickel-based active material, whereinthe lithium secondary battery has an increased discharge capacity,improved charge/discharge efficiency, and a long lifespan.

Additional aspects of embodiments will be set forth in part in thedescription which follows and, in part, will be apparent from thedescription, or may be learned by practice of the presented embodiments.

According to one or more embodiments, a nickel-based active material fora lithium secondary battery includes at least one secondary particlethat includes at least two primary particle structures, the primaryparticle structures each including a porous inner portion and an outerportion including a radially arranged structure, and the secondaryparticle including at least two radial centers.

According to one or more embodiments, a method of preparing anickel-based active material for a lithium secondary battery includesprimary heat-treating a mixture of a lithium precursor and a metalhydroxide in an oxidative gas atmosphere at a temperature in a range ofabout 600° C. to about 850° C.; and secondary heat-treating the mixturein an oxidative gas atmosphere at a temperature in a range of about 700°C. to about 950° C. to prepare the nickel-based active material.

According to one or more embodiments, a lithium secondary batterycontains a positive electrode including the nickel-based active materialfor a lithium secondary battery.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects of embodiments of the present disclosure willbecome apparent and more readily appreciated from the followingdescription of the embodiments, taken in conjunction with theaccompanying drawings in which:

FIG. 1A is a schematic view that illustrates a structure of anickel-based active material according to an embodiment;

FIG. 1B is a schematic view that illustrates a plate particleconstituting primary particles of the nickel-based active material shownin FIG. 1A;

FIG. 1C is a schematic view of a structure of a cross-section takenthrough centers of secondary particles of a nickel-based active materialaccording to an embodiment;

FIG. 1D is a schematic view of shapes of embodiments of a plateparticle;

FIG. 1E is a view that illustrates a definition of a radial-type (orkind) of arrangement of a secondary particle of the nickel-based activematerial according to an embodiment;

FIG. 2 is a schematic view of a lithium secondary battery according toan embodiment;

FIG. 3A is a scanning electron microscope (SEM) image of nickel-basedactive material prepared in Example 1;

FIG. 3B is a SEM image of nickel-based active materials prepared inComparative Example 1;

FIG. 3C is a SEM image of nickel-based active materials prepared inComparative Example 2;

FIG. 4 is a graph that shows a voltage change according to an initialcapacity in each of coin cells prepared in Manufacture Example 1 andComparative Manufacture Examples 1 and 2;

FIG. 5 is a graph that shows a change in discharge capacity according tothe number of cycles in each of coin cells prepared in ManufactureExample 3 and Comparative Manufacture Examples 3 and 4;

FIG. 6 is a graph that shows a change in discharge capacity according tothe number of cycles in each of full cells prepared in ManufactureExample 3 and Comparative Manufacture Examples 3 and 4; and

FIG. 7 is a schematic view that illustrates a comparison of migrationdistances of lithium ions of a nickel-based active material B accordingto an embodiment with migration distances of lithium ions of anothernickel-based active material A.

DETAILED DESCRIPTION

Reference will now be made in more detail to embodiments of the presentdisclosure, examples of which are illustrated for a nickel-based activematerial for a lithium secondary battery, a method of preparing thenickel-based active material, and a lithium secondary battery includinga positive electrode including the nickel-based active material in theaccompanying drawings, wherein like reference numerals refer to likeelements throughout. In this regard, the present embodiments may havedifferent forms and should not be construed as being limited to thedescriptions set forth herein. Accordingly, the embodiments are merelydescribed below, by referring to the figures, to explain aspects ofembodiments of the present description. As used herein, the term“and/or” includes any and all combinations of one or more of theassociated listed items. Expressions such as “at least one of,” whenpreceding a list of elements, modify the entire list of elements and donot modify the individual elements of the list.

Hereinafter, referring to FIGS. 1A-1E, the nickel-based active materialfor a lithium secondary battery, according to the present disclosure,will be described in more detail. FIG. 1A is a schematic view thatillustrates a structure of a nickel-based active material according toan embodiment. FIG. 1B is a schematic view that illustrates a plateparticle constituting primary particles of a nickel-based activematerial shown in FIG. 1A. FIG. 1C is a schematic view of a structure ofa cross-section taken through centers of secondary particles of anickel-based active material according to an embodiment.

The nickel-based active material for a lithium secondary battery mayinclude at least one secondary particle that includes at least twoprimary particle structures, wherein the primary particle structure mayinclude a porous inner portion and an outer portion including a radiallyarranged structure, and wherein the secondary particle may include atleast two radial centers.

In the nickel-based active material for a lithium secondary battery, theprimary particle may be positioned in a multicenter isotropicarrangement, thus forming a secondary particle.

As used herein, the term “primary particle structure” refers to anagglomerate of at least two primary particles.

Referring to FIG. 1A, a secondary particle 12 has a structure in which aplurality of primary particle structures 11 are isotropically arranged.The primary particle structures 11 include a porous inner portion 11 aand an outer portion 11 b including a radially arranged structure. Theprimary particle structures 11 may be an agglomerate formed by isotropicarrangement of at least two primary particles 11 c. The primary particlestructures 11 may include a nickel-based active material.

The primary particle structures 11 may be radially arranged in at leastone part, and the secondary particle 12 may include at least two primaryparticle structures 11. The primary particle structures 11 may have adiameter (or width) i, and the secondary particle 12 may have a diameter(or width) ii.

As shown in FIG. 1A, the outer portion 11 b of the primary particlestructure 11 may have, for example, a structure in which plateparticles, e.g., the primary particles 11 c, are radially arranged. Atleast two such primary particles 11 c may be aligned in an isotropicarrangement (or a radial arrangement) to form the primary particlestructure 11, and at least two primary particle structures 11 may bealigned in an isotropic arrangement to form the secondary particle 12.

As used herein, the term “radial center” denotes, as shown in FIG. 1A,the center of the primary particle structure 11 containing the porousinner portion 11 a and the outer portion 11 b including a radiallyarranged structure.

In some embodiments, a sphericity of the most frequent particle of thenickel-based active material is in a range of about 0.960 to about0.985, for example, about 0.970 to about 0.980. Also, a sphericity ofparticles in a positive electrode is in a range of about 0.960 to about0.985, for example, about 0.970 to about 0.975. When the nickel-basedactive material and the positive electrode have the sphericity withinthese ranges, a lithium secondary battery with excellent dischargecapacity characteristics and improved charge/discharge efficiency may bemanufactured. A size of the primary particle structure 11 may be in arange of about 2 μm to about 5 μm, for example, about 3 μm to about 4μm, and a size of the nickel-based active material secondary particle 12may be in a range of about 5 μm to about 25 μm, for example, about 5 μmto about 22 μm, for example, about 7 μm to about 20 μm, for example,about 9 μm to about 15 μm. As shown in FIG. 1B, plate particles, e.g.,the primary particles 11 c, constituting the primary particle structure11, may have a ratio of horizontal length iii to a vertical length iv ina range of about 1:2 to about 1:20, for example, about 1:3 to about1:20, or, for example, about 1:5 to about 1:15. When sizes of theprimary particle structure 11 and the primary particle 11 c are withinthese ranges, use of lithium ions in the nickel-based active materialmay be facilitated.

FIG. 1C is a schematic view of a structure of a cross-section takenthrough centers of primary particle structures of a secondary particle12 of a nickel-based active material according to an embodiment and anexpanded view (A) of a plate particle, e.g., the primary particle 11 c.As shown in FIG. 1C, an embodiment of the plate particle 11 c has alength L, a thickness T, and a substantially hexagonal shape in a planview.

As used herein, the term “size” refers to an average particle diameterwhen particles are spherical or an average length of a long axis whenparticles are not spherical. A size of the particles may be measured byusing a particle size analyzer (PSA).

In some embodiments, a pore size of the porous inner portion 11 a in thenickel-based active material may be in a range of about 150 nm to about1 μm, for example, about 150 nm to about 800 nm, for example, about 200nm to about 550 nm, and a pore size of the outer portion 11 b may beless than about 150 nm, for example, about 100 nm or less, for example,in a range of about 20 nm to about 90 nm. As used herein, the term “apore size” refers to an average diameter of a pore or a width of anopening of a pore when the pore is spherical or circular. When the poreis non-spherical or non-circular, such as in an elliptical form, thepore size refers to an average length of a long axis of the opening ofthe pore.

In the primary particle structure 11, a porosity of the porous innerportion 11 a may be in a range of about 5% to about 15%, for example,about 5% to about 10%, and a porosity of the outer portion 11 b may bein a range of about 1% to about 5%, for example about 1% to about 3%.When the pore size and porosity of the primary particle structure 11 iswithin these ranges, a nickel-based active material having improvedcapacity characteristics may be obtained.

In some embodiments, a porosity of the outer portion 11 b of the primaryparticle structure 11 may be controlled to be smaller than that of theinner portion 11 a. The pore size and porosity of the inner portion 11 amay be irregular and greater than those of outer portion 11b. When theporosity of the inner portion 11 a and the outer portion 11 b of theprimary particle structure 11 is within these ranges as described above,a degree of tightness in the outer portion 11 b is higher than that ofthe inner portion 11 a, which may result in effective reduction orsuppression of a side reaction with an electrolyte solution.

In the inner portion 11 a of the primary particle structure 11, a closedpore may be present, and a closed and/or open pore may be present in theouter portion 11 b. It may be difficult to contain electrolyte or thelike in a closed pore, whereas it may be possible to contain electrolyteor the like inside an open pore of the primary particle structure 11. Inaddition, the primary particle structure 11 may have irregular porouspore (e.g., an irregular porous structure) in the inner portion 11 a.The inner portion 11 a including the irregular porous structure mayinclude plate particles as well as the outer portion 11 b, and plateparticles in the inner portion 11 a may be irregularly aligned, unlikethe outer portion 11 b, which may be radially aligned.

As used herein, the term “irregular porous pore” and “irregular porousstructure” refer to a pore (or pore structure) that may have irregularpore sizes and shapes and do not have uniformity. Unlike the outerportion 11 b, the inner portion 11 a including the irregular porous pore(e.g., the irregular porous structure) may include amorphous particles.Unlike the outer portion 11 b, the amorphous particles are arrangedwithout regularity.

When the nickel-based active material according to an embodiment has astructure described above, capacity characteristics of the nickel-basedactive material may be stable as compared to those of a nickel-basedactive material having substantially the same composition except withoutthe features described herein. Also, when the number of radial centersis at least two, migration distances of lithium ions from a surface tothe center of the secondary particles decrease (e.g., absolutelydecrease), and thus availability of lithium ions may increase.

FIG. 7 is a schematic view that illustrates lithium migration distancesof lithium ions of a nickel-based active material B according to anembodiment and a general nickel-based active material A (for example, anickel-based active material of Comparative Example 1). Referring toFIG. 7, a lithium ion migration distance L2 from a surface portion tothe center of the nickel-based active material B according to anembodiment is shorter than a lithium ion migration distance L1 from asurface portion to the center of the general nickel-based activematerial A. Therefore, when an availability of lithium increases, acapacity of the nickel-based active material according to an embodimentmay improve even when an amount of nickel is not increased.

In the nickel-based active material according to an embodiment, poresexist in the nickel-based active material, and thus cracks of particlescaused by the volume change that occurs during a charge/dischargeprocess may be prevented (or a likelihood, degree, or amount of suchcracks may be reduced), which may prevent or reduce an increase ofresistance. In this regard, long lifespan characteristics of a lithiumsecondary battery including the nickel-based active material mayimprove.

A multicenter radial arrangement may have about 10% to about 50%, forexample, about 20% to about 40%, of an isotropic arrangement area on across-section that passes through a center of a secondary particle ofthe most frequent particle (e.g., the most abundant particle). Particlesother than those in the isotropic arrangement area are aligned in ananisotropic arrangement. Particles in a general radial arrangement existin the anisotropic arrangement area. As used herein, the term “the mostfrequent particle” denotes a group of particle shapes that is the mostabundant in the nickel-based active material.

A laser diffraction/scatter type particle size distribution measuringdevice (SALD-2100, manufactured by Shimadzu Corporation) may be used tomeasure the particle size distribution. Then, the volume averageparticle diameter, each of the maximal volume frequencies, D10, D50,D90, D99, and the most frequent particle size are calculated from themeasured particle size distribution. The most frequent particle size isa so-called mode diameter, which is the most frequent particle size inthe particle size distribution, i.e., a particle size of particles thatare the highest amount.

As used herein, the term “plate particle” refers to a particle having athickness that is smaller than a length of a longer axis (in a planedirection). Here, the length of the longer axis refers to an upperlength (e.g., a maximum length) of the widest plane of the plateparticle.

The plate particle may refer to a structure in which a length (orthickness) t in one axial direction (e.g., a thickness direction) isshorter than a length of a longer axis a in a direction (e.g., a planedirection) different from the one axial direction.

FIG. 1D is a schematic view of shapes of embodiments of a plate particleaccording to an embodiment.

Referring to FIG. 1D, the plate particle may have a polygonal nanoplateshape such as, for example, a hexagon (A), a nanodisc shape (B), and arectangular parallelepiped shape (C).

In FIG. 1D, a thickness t of the plate particle is smaller than lengthsa and b in a plan view of the plate particle. Here, the length a in aplane direction (or plan view) may be longer than or equal to the lengthb in a plane direction (or plan view). Regarding a plate particle, adirection in which the thickness t is defined is referred to as “athickness direction”, and a direction in which the length a or b isdefined is referred to as “a plane direction”.

The term “radial” used herein refers to an arrangement as shown in FIG.1 E in which the direction of the thickness t is aligned to beperpendicular (e.g., substantially perpendicular) to or aligned to be85° to 95° a with respect to direction R heading to the center of thesecondary particle. For example, as shown in FIG. 1E, plate particlesmay be radially arranged to have the plane direction aligned along thedirection R such that the thickness t may be perpendicular (e.g.,substantially perpendicular) to the direction R, one selected from thelengths a and b is substantially parallel to the direction R, and theother of the lengths a and b is perpendicular or substantiallyperpendicular to the plane in which R is arranged.

Definitions of the terms “inner portion” and “outer portion” of anactive material, as used herein, will now be described in more detail.

The term “outer portion,” as used herein, refers to an area occupying30% to 50% (for example, 40%) of a length from an outer surface withrespect to a total distance between the center and a surface of thenickel-based active material, or an area within 2 μm from the outmostperiphery (e.g., the outer surface) of the nickel-based active material.The term “inner portion,” as used herein, refers to an area occupying50% to 70% (for example, 60%) of a length from the center with respectto a total distance between the center and a surface of the nickel-basedactive material, or an area outside an area within 2 μm from the outmostperiphery (e.g., the outer surface) of the nickel-based active material.

As used herein, the term “isotropic arrangement” refers to anarrangement of particles in which a property of the particles does notchange even when a direction of observation of matter formed of theparticles changes. A direction of the arrangement used herein may not beknown (e.g., may not be set), and a “multicenter” reduces a distance forlithium ion migration from a surface of a particle to the center thereofas the particle has at least two centers. With the distance thusreduced, resistance decreases accordingly, and thus the term“multicenter” used herein refers to a structure effective in terms ofcharge/discharge efficiency and a long lifespan.

An average thickness of the plate particles constituting the outerportion is in a range of about 100 nm to about 250 nm, or, for example,about 100 nm to about 200 nm, and an average length of a long axis ofthe plate particles is in a range of about 250 nm to about 1100 nm, or,for example, about 300 nm to about 1000 nm. Here, the average length ofthe long axis of the plate particle is about 2 to 10 times the averagethickness. When the average length, the average thickness, and the ratioof the average thickness to the average length of the plate particle aresatisfied within the ranges above, the size of the plate particles issmall, and the primary particles are arranged radially in the outerportion of the secondary particle of the nickel-based active material,relatively many lithium diffusion paths between grain boundaries may beexposed on the surface of the secondary particle of the nickel-basedactive material, and many crystal surfaces capable of transferringlithium to the outer portion of the secondary particle of thenickel-based active material may be exposed.

The nickel-based active material may include an active materialrepresented by Formula 1:

Li_(a)(Ni_(1-x-y-z)CO_(x)Mn_(y)M_(z))O₂   Formula 1

In Formula 1, M is an element selected from the group consisting ofboron (B), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba),titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu),zirconium (Zr), and aluminum (Al); and

x, y, and z satisfy the following relations: 0.95≤a≤1.3, x≤(1x-y-z),y≤(1-x-y-z), 0<x<1, 0≤y<1, 0≤z<1, and 0<(1-x-y-z)<1. In the nickel-basedactive material of Formula 1, the amount of nickel in the nickel-basedactive material may be in a range of about 30 mol % to about 95 mol %based on a total amount of transition metals including nickel, cobalt,and manganese. The amount of nickel may be greater than an amount ofcobalt, and an amount of nickel may be greater than an amount ofmanganese. In Formula 1, a, x, y, and z satisfy the following relations:0.95≤a≤1.3, for example, 1.0≤a≤1.1, 0<x≤1/3, for example, 0.1≤a≤1/3,0≤y≤0.5, for example, 0.05≤y≤0.3, 0≤z≤0.05, and 1/3(1-x-y-z)≤0.95. In anembodiment, in Formula 1, a, x, y, and z satisfy the followingrelations: 0.95≤a≤1.3, for example, 1.0≤a≤1.1; 0<x≤1/3, for example,0.1≤x≤1/3; 0≤y≤0.5, for example, 0.05≤y≤0.3; 0≤z≤0.05; and1/3(1-x-y-z)≤0.95.

In one embodiment, z in Formula 1 is 0.

In some embodiments, when z satisfies 0<z≤0.05 in Formula 1, M may bealuminum.

The amount of Ni in the nickel-based active material may be greater thanthat of each of other transition metals, based on 1 mole of totaltransition metals. When the nickel-based active material having such alarge amount of Ni is used for a positive electrode to be included in alithium secondary battery, the lithium secondary battery may have highlithium diffusion degree, good conductivity, and high capacity atsubstantially the same voltage. However, due to cracks generatedtherein, lifespan of the lithium secondary battery may be degraded.

The nickel-based active material may includeLiNi_(0.6)Co_(0.2)Mn_(0.2)O₂, LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂,LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂, and/orLiNi_(0.85)Co_(0.1)Al_(0.0.05)O₂.

The method of preparing a nickel-based active material according to anembodiment may include a primary heat-treatment of a lithium precursorand metal hydroxide and a secondary heat-treatment.

The primary heat-treatment may include mixing a lithium precursor with ametal hydroxide at a stoichiometric or constant molar ratio, and then,heat-treatment (e.g., low-temperature heat-treatment) is performed onthe mixture at a temperature in a range of about 600° C. to about 850°C.

The metal hydroxide may be a compound represented by Formula 2:

(Ni_(1-x-y-z)Co_(x)Mn_(y)Mz)(OH)₂   Formula 2

In Formula 2, M may be an element selected from the group consisting ofB, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Cu, Zr, and Al, and

x, y, and z satisfy the following relations: x≤(1-x-y-z), y≤(1-x-y-z),0<x<1, 0≤y<1, 0≤z<1, and 0<(1-x-y-z)<1.

In Formula 2, x, y, and z satisfy 0<x≤1/3, 0≤y≤0.5, 0≤z≤0.05, and1/3≤(1-x-y-z)≤0.95.

In Formula 2, x, y, and z satisfy 0.5≤(1-x-y-z)≤0.95.

In Formula 2, the metal hydroxide may include, for example,Ni_(0.6)Co_(0.2)Mn_(0.2)(OH)₂, Ni_(0.5)Co_(0.2)Mn_(0.3)(OH)₂,Ni_(1/3)Co_(1/3)Mn_(1/3)(OH)₂, and/or Ni_(0.8)Co_(0.1)Mn_(0.1)(OH)₂.

The lithium precursor may include, for example, lithium hydroxide,lithium fluoride, lithium carbonate, or a mixture thereof. A ratio atwhich the lithium precursor and the metal hydroxide are mixed may bestoichiometrically adjusted to prepare the metal hydroxide of Formula 2.

Here, the mixing may be dry mixing, or may be performed using a mixer orthe like.

The drying mixing may be performed according to a milling method. Here,in consideration of milling conditions, the drying mixing may beperformed in a way such that metal hydroxide used as a starting materialbarely undergoes deformation such as pulverization. In this regard, aprocess of controlling the size of the lithium precursor, which is to bemixed with the metal hydroxide, in advance may be performed. In anembodiment, the size (e.g., particle size) of the lithium precursor maybe in a range of about 5 μm to about 15 μm, for example, may be about 10μm. When the lithium precursor having the size (e.g., particle) withinthe range above is subjected to a milling process with the metalhydroxide, a desired nickel-based active material may be obtained

In one embodiment, the size (e.g., particle size) of the metal hydroxidemay be almost identical to that of the nickel-based active material. Inan embodiment, the etal hydroxide which is a composite metal hydroxidemay have, for example, an average thickness in a range of about 100 nmto about 250 nm, an average length in a range of about 250 nm to about1,100 nm, an internal pore size in a range of about 150 nm to about 550nm, and an external pore size in a range of about 50 nm to about 148 nm.

The primary heat-treatment may be performed in an oxidative gasatmosphere. In the oxidative gas atmosphere, oxidative gas, such as, forexample oxygen or air, may be used.

The primary heat-treatment may be suitably or appropriately performed ina range of densification temperature or lower as the reaction of thelithium precursor and the metal hydroxide proceeds. Here, thedensification temperature refers to a temperature at which suitable orsufficient crystallization may be performed to realize the upper (e.g.,maximum) charging capacity of an active material.

The primary heat-treatment may be performed, for example, at atemperature in a range of about 600° C. to about 850° C., or about 650°C. to about 800° C. Here, the time for the primary heat-treatment mayvary according to the temperature at which the primary heat-treatment isperformed. For example, the primary heat-treatment may be performed forabout 3 hours to about 10 hours.

The secondary heat-treatment may include a process, in which thesecondary particles of the nickel-based active material may be subjectedto a heat-treatment in an oxidative gas atmosphere where an exhaust ofthe secondary particles is suppressed or reduced.

When the exhaust of the secondary particles is suppressed or reduced inmanufacturing, the generation of the surface resistive layer may besuppressed or reduced as much as possible or practical by maintainingthe atmosphere inside a reactor and particle densification may beperformed.

The secondary heat-treatment may be performed at a temperature in arange of about 700° C. to about 950° C. The time for the secondaryheat-treatment may vary according to the temperature at which thesecondary heat-treatment is performed. For example, the secondaryheat-treatment may be performed for about 3 hours to about 10 hours. Amethod of preparing a metal hydroxide that is used in the preparation ofthe nickel-based active material may be described as follows.

Such a method of preparing the metal hydroxide is not particularlylimited, but for example, may be performed according to aco-precipitation method and/or a solid phase method.

Hereinafter, as an example for the metal hydroxide, the compound ofFormula 2 is subjected to a co-precipitation method to describe anembodiment of the method of preparing the metal hydroxide.

A raw material for the nickel-based active material, such as a Niprecursor, a Co precursor, a Mn precursor, and a metal M precursor, ismixed with a solvent to obtain a precursor mixture.

Here, an amount of the Ni precursor, the Co precursor, the Mn precursor,and the metal M precursor may be stoichiometrically adjusted to preparethe compound of Formula 2.

Examples of the solvent include water, ethanol, propanol, and butanol.Also, an amount of the solvent may be in a range of about 100 parts toabout 2000 parts by weight based on 100 parts by weight of the totalweight of the Ni precursor, the Co precursor, the Mn precursor, and themetal M precursor.

A precipitating agent and a pH regulator may be added to the precursormixture to control pH of the resulting mixture, and a co-precipitationmethod is performed thereon to obtain precipitations (e.g.,precipitates). Here, pH of the resulting mixture is adjusted to pH 11 topH 13. The precipitations (e.g., precipitates) obtained therefrom aresubjected to filtration and heat treatment. Here, the heat treatment isperformed at a temperature in a range of about 20° C. to about 160° C.to dry the filter cakes (e.g., filtrates obtained from filtration andheat treatment of the precipitates).

The precipitating agent may serve to control a reaction (formation) rateof the precipitations in the co-precipitation reaction, and examplesthereof include ammonium hydroxide (NH₄OH) and citric acid. Here, anamount of the precipitating agent may be used at any suitable amount,such as those generally used in the art.

The pH regulator may serve to control pH of a reaction mixture to be ina range of pH 11 to pH 13, and examples thereof include ammoniumhydroxide (NH₄OH), sodium hydroxide (NaOH), sodium carbonate (Na₂CO₃),and sodium oxalate (Na₂C₂O₄)

Examples of the Ni precursor include nickel sulfate, nickel chloride,and nickel nitrate. Examples of the Co precursor include cobalt sulfate,cobalt chloride, and cobalt nitrate. Examples of the Mn precursorinclude manganese sulfate, manganese nitrate, and manganese chloride.Examples of the metal M precursor include metal carbonate, metalsulfate, metal nitrate, and metal chloride.

Hereinafter, a method of preparing a lithium secondary battery includinga positive electrode including the nickel-based active materialaccording to an embodiment, a negative electrode, a non-aqueouselectrolyte containing a lithium salt, and a separator will bedescribed.

A positive electrode and a negative electrode are prepared by applying acomposition for forming a positive active material layer and acomposition for forming a negative active material layer on a currentcollector, respectively, and then, by drying the current collector.

The composition for forming the positive active material layer may beprepared by mixing a positive active material, a conductive agent, abinder, and a solvent. Here, a positive active material according to anembodiment may be used as the positive active material.

The binder, which is a component that assists in binding between acurrent collector with an active material, a conductive agent, or thelike, may be added in an amount in a range of about 1 part to about 50parts by weight based on 100 parts by weight of the total weight of thepositive active material. Non-limiting examples of the binder arepolyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose,polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene,ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrenebutadiene rubber, fluorine rubber, and various suitable copolymers.

As the conductive agent, any suitable material may be used as long as ithas electrical conductivity without causing an unsuitable, undesirable,and/or unwanted chemical change in a corresponding battery including theconductive agent. Examples of the conductive agent include graphite,such as, for example, natural graphite or artificial graphite; acarbonaceous material, such as, for example, carbon black, acetyleneblack, ketjen black, channel black, furnace black, lamp black, or summerblack; conductive fiber, such as, for example, carbon fiber or metalfiber; fluorocarbon;

metal powder, such as, for example, aluminum powder or nickel powder;conductive whisker, such as, for example, zinc oxide or potassiumtitanate; and a conductive material, such as, for example, apolyphenylene derivative.

A non-limiting example of the solvent includes N-methylpyrrolidone.

The binder, conductive agent, and solvent may be used in an amount thatis generally used in the art.

For use as a positive current collector, a material is not particularlylimited as long as it has a thickness in a range of about 3 μm to about500 μm and has high conductivity without causing an unsuitable,undesirable, and/or unwanted chemical change in a corresponding batteryincluding the positive current collector. Examples of the positivecurrent collector include stainless steel, aluminum, nickel, titanium,heat treated carbon, and aluminum or stainless steel that is surfacetreated with carbon, nickel, titanium, or silver. The current collectormay have a fine uneven structure at its surface to increase a bindingforce with respect to the positive active material. The currentcollector may have various suitable shapes, such as, for example, film,sheet, foil, net, porous, foam, or non-woven shape.

In a separate manner, the composition for forming the negative activematerial may be prepared by mixing a negative active material, thebinder, the conductive agent, and the solvent. As the negative activematerial, a material that allows lithium ions to intercalate thereintoor deintercalate therefrom may be used. Non-limiting examples of thenegative active material include graphite, a carbonaceous material, suchas carbon, lithium metal, an alloy thereof, and a silicon oxide-basedmaterial.

An amount of the binder may be in a range of about 1 part to about 50parts by weight based on 100 parts by weight of the total weights of thenegative active material. Non-limiting examples of the binder may besubstantially the same as those described above in connection with thepositive electrode.

An amount of the conductive agent may be in a range of about 1 part toabout 5 parts by weight, based on 100 parts by weight of the totalweights of the negative active material. When the amount of theconductive agent is within the range above, a finally obtained negativeelectrode may have excellent conductivity.

An amount of the solvent may be in a range of about 100 to about 3000parts by weight, based on 100 parts by weight of the total weights ofthe negative active material. When the amount of the solvent is withinthe range above, the negative active material layer may be easilyformed.

The conductive agent and the solvent may be substantially the same asthose described above in connection with the positive electrode.

The negative current collector may have a thickness in a range of about3 μm to about 500 μm. As the negative current collector, any suitablematerial may be used as long as it has electrical conductivity withoutcausing an unsuitable, undesirable, and/or unwanted chemical change in acorresponding battery including the negative current collector. Examplesof the negative current collector include copper, stainless steel,aluminum, nickel, titanium, heat treated carbon, and copper or stainlesssteel that is surface treated with carbon, nickel, titanium, silver, andan aluminum-cadmium alloy. In addition, like the positive currentcollector, the negative current collector may have a fine unevenstructure at its surface to increase a binding force thereof withrespect to a negative active material. For example, the negative currentcollector may have various suitable shapes, such as, for example, film,sheet, foil, net, porous, foam, or non-woven shape.

A separator may be disposed between the positive electrode and thenegative electrode that are manufactured as described above.

The separator may have a pore diameter in a range of about 0.01 μm toabout 10 μm and a thickness in a range of about 5 μm to about 300 μm.Examples of the separator include an olefin-based polymer, such as, forexample, polypropylene or polyethylene; or a sheet or non-fabric formformed of glass fiber. When a solid electrolyte, such as, for example, apolymer, is used as the electrolyte, the solid electrolyte may also actas a separator.

The non-aqueous electrolyte containing a lithium salt may include anon-aqueous electrolyte and a lithium salt. Examples of the non-aqueouselectrolyte include an organic solid electrolyte and an inorganic solidelectrolyte.

Non-limiting examples of the non-aqueous electrolyte include an aproticorganic solvent, such as, for example, N-methyl-2-pyrrolidinone,propylene carbonate, ethylene carbonate, butylene carbonate, dimethylcarbonate, diethyl carbonate, gammabutyrolactone, 1,2-dimethoxy ethane,2-methyl tetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide,N,N-dimethylformamide, acetonitrile, nitromethane, methyl formate,methyl acetate, triester phosphate, trimethoxy methane, sulfolane,methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, a tetrahydrofuranderivative, ether, methyl propionate, and ethyl propionate.

Non-limiting examples of the organic solid electrolyte include apolyethylene derivative, a polyethylene oxide derivative, apolypropylene oxide derivative, polyvinyl alcohol, and polyvinylidenefluoride.

Non-limiting examples of the inorganic solid electrolyte include Li₃N,Lil, Li₅NI₂, Li₃N—LiI—LiOH, Li₂SiS₃, Li₄SiO₄, Li₄SiO₄—LiI—LiOH, orLi₃PO₄—Li₂S—SiS₂.

The lithium salt may be any one of various suitable materials that areeasily dissolved in the non-aqueous electrolyte, and non-limitingexamples thereof include LiCl, LiBr, Lil, LiCIO₄, LiBF₄, LiB₁₀Cl₁₀,LiPF₆, LiCF₃SO₃, LiCF₃CO₂, LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li,(CF₃SO₂)₂NLi, (FSO₂)₂NLi, lithium chloroborate, lithium lower aliphaticcarboxylate, and lithium tetrakisphenyl borate.

FIG. 2 is a schematic view of a structure of a lithium secondary batteryaccording to an embodiment.

Referring to FIG. 2, the lithium secondary battery 21 includes apositive electrode 23, a negative electrode 22, and a separator 24. Thepositive electrode 23, the negative electrode 22, and the separator 24may be wound or folded, and then sealed in a battery case 25. Then, thebattery case 25 may be filled with an organic electrolyte and sealedwith a cap assembly 26, thereby completing the manufacture of thelithium secondary battery 21. The battery case 25 may be a cylindricaltype (or kind), a rectangular type (or kind), or a thin-film type (orkind). For example, the lithium secondary battery 21 may be a thin-filmtype (or kind) of battery. For example, the lithium secondary battery 21may be a lithium ion battery.

The separator 24 may be disposed between the positive electrode 23 andthe negative electrode 22 to form a battery assembly. The batteryassembly may be stacked in a bi-cell structure and impregnated with theorganic electrolyte. The resultant assembly may be put into a pouch andhermetically sealed, thereby completing the manufacture of a lithium ionpolymer battery. In addition, a plurality of battery assemblies may bestacked to form a battery pack, which may be used in any suitable devicethat requires high capacity and high output, for example, in a laptopcomputer, a smart phone, or an electric vehicle.

In addition, the lithium secondary battery may have excellent storagestability at a high temperature, improved lifetime characteristics, andhigh rate characteristics, and thus may be used in an electric vehicle(EV), for example, in a hybrid vehicle such as a plug-in hybrid electricvehicle (PHEV).

The following Examples and Comparative Examples are provided in order tofurther describe the subject matter of the present disclosure, but itwill be understood that the Examples and Comparative Examples are not tobe construed as limiting the scope of the embodiments, nor are theComparative Examples to be construed as being outside the scope of theembodiments.

EXAMPLE 1

A radial and porous metal hydroxide (Ni_(0.6)Co_(0.2)Mn_(0.2)(OH)₂) anda lithium hydroxide (LiOH) were dry-mixed at a molar ratio of 1:1 toprepare a mixture. The mixture was heat-treated in an oxygen atmosphereat about 700° C. for 6 hours to prepare a nickel-based active material(LiNi_(0.6)Co_(0.2)Mn_(0.2)O₂). The nickel-based active material thusprepared had a porous structure in an inner portion and a radiallyarranged structure in an outer portion. The nickel-based active materialwas heat-treated in the air at about 800° C. for 6 hours to prepare anickel-based active material secondary particle(LiNi_(0.6)Co_(0.2)Mn_(0.2)O₂).

The metal hydroxide (Ni_(0.6)Co_(0.2)Mn_(0.2)(OH)₂) metal precursor wasprepared as follows.

A nickel sulfate (NiSO₄.6H₂O), a cobalt sulfate (CoSO₄.7H₂O), and amanganese sulfate (MnSO₄.H₂O), as metal precursors for forming anickel-based active material, were dissolved at a molar ratio of 6:2:2in a solvent, that is, distilled water, such that a total concentrationof the metal raw materials was 2 M, and thus a mixed metal sourcesolution was prepared. In consideration of formation of a complexcompound, 29 wt % of ammonia water (NH₄OH) and 6M of sodium hydroxide(NaOH), as a precipitating agent, were used. The mixed metal sourcesolution and ammonia water were added to a reactor at a speed of 5.0L/hr and 0.5 L/hr, respectively, and sodium hydroxide was then addedthereto to adjust pH to about 11 to 12. The stirring power wascontrolled to about 2.5 kW/m³ to about 6.0 kW/m³. The reaction wascontinued until seeds having a size of about 40% to about 60% of a sizeof final positive active material particles were grown. Subsequently,the adding speeds of the mixed metal source solution and the ammoniawater were changed to about 6.5 L/hr and 1.5 L/hr, respectively to growthe seeds for the particles having an average particle diameter (D50) ofabout 9 μm to about 11 μm, while sodium hydroxide was added to adjust pHto about 10 to 11, and the stirring power was controlled to about 2.0kW/m³ to 2.5 kW/m³.

Subsequently, the adding speeds of the mixed metal source solution andthe ammonia water were changed to about 7.5 L/hr and 2.0 L/hr, whilesodium hydroxide was added to adjust pH to about 10 to 10.5, and thestirring power was controlled to about 1.5 kW/m³ to 2.0 kW/m³. Thereaction was continued until the particles having an average particlediameter (D50) of about 10 μm to 12 μm were formed. That is, a precursorwas obtained by stepwise decreasing the stirring power and pH andstepwise increasing the source supply speed.

A slurry solution in the reactor was filtered and washed withhigh-purity distilled water, and then, dried in a hot-air oven at 150°C. for 24 hours, thereby obtaining a metal hydroxide precursor(Ni_(0.6)Co_(0.2)Mn_(0.2)(OH)₂) powder.

EXAMPLE 2

A nickel-based active material particle was obtained in substantiallythe same manner as in Example 1, except for doubling the reaction timeso that the reaction was carried out at 50° C. for about 26 hoursinstead of carrying out the reaction at 50° C. for about 13 hours in thepreparation of the metal hydroxide.

COMPARATIVE EXAMPLE 1

A metal hydroxide (Ni_(0.6)Co_(0.2)Mn_(0.2)(OH)₂) and a lithiumhydroxide (LiOH) were dry-mixed at a molar ratio of 1:1 to prepare amixture. The mixture was heat-treated at about 800° C. for 12 hours toprepare a nickel-based active material (LiNi_(0.6)Co_(0.2)Mn_(0.2)O₂). Aparticle of the nickel-based active material thus prepared did not havepores in an inner portion, but amorphous primary particles in the innerportion and the outer portion had an isotropically arranged structure.

The metal hydroxide (Ni_(0.6)Co_(0.2)Mn_(0.2)(OH)₂) was prepared asfollows. A nickel sulfate (NiSO₄.6H₂O), a cobalt sulfate (CoSO₄.7H₂O),and a manganese sulfate (MnSO₄.H₂O), as metal precursors for forming anickel-based active material, were dissolved at a molar ratio of 6:2:2in distilled water as a solvent, so that a total concentration of themetal raw materials (nickel sulfate, a cobalt sulfate, and a manganesesulfate) was 1.3 M, and thus a mixed solution was prepared. Inconsideration of formation of a composite compound, 29 wt % of ammoniawater (NH₄OH) and 6 M of sodium hydroxide (NaOH), as a precipitatingagent, were used.

A mixed solution of metal raw materials, ammonia water at an amount of 1M with respect to the metal raw materials, and sodium hydroxide wereeach continuously added to the reactor containing diluted ammonia water.The amount of ammonia water was 1 mol based on 1 mol of the metal rawmaterial. Next, the reaction was carried out at 50° C. for about 24hours while the reaction mixture in the reactor was stirred.

A slurry solution in the reactor was filtered and washed withhigh-purity distilled water, and then, dried in a hot-air oven at 150°C. for 24 hours, thereby obtaining a metal hydroxide precursor(Ni_(0.6)Co_(0.2)Mn_(0.2)(OH)₂) powder.

COMPARATIVE EXAMPLE 2

A radial composite metal hydroxide (Ni_(0.6)Co_(0.2)Mn_(0.2)(OH)₂) and alithium hydroxide (LiOH) were dry-mixed at a molar ratio of 1:1 toprepare a mixture. The mixture was heat-treated at about 700° C. for 12hours to prepare a nickel-based active material(LiNi_(0.6)Co_(0.2)Mn_(0.2)O₂). A particle of the nickel-based activematerial thus prepared did not have pores in an inner portion, butamorphous primary particles in the inner portion and the outer portionhad a radially arranged structure.

The radial composite metal hydroxide (Ni_(0.6)Co_(0.2)Mn_(0.2)(OH)₂) wasprepared as follows. A nickel sulfate (NiSO₄.6H₂O), a cobalt sulfate(CoSO₄.7H₂O), and a manganese sulfate (MnSO₄.H₂O), as metal precursorsfor forming a nickel-based active material, were dissolved at a molarratio of 6:2:2 in distilled water as a solvent, so that a totalconcentration of the metal raw materials was 1.3 M, and thus a metalprecursor mixture was prepared. In consideration of formation of acomposite compound, ammonia water (NH₄OH) and sodium hydroxide (NaOH),as a precipitating agent, were used.

A mixed solution of metal raw materials, ammonia water at an amount of 1mol as a molar ratio with respect to the metal raw materials, and sodiumhydroxide were each continuously added to the reactor containing dilutedammonia water. The amount of ammonia water (ammonium hydroxide) was 1mol based on 1 mol of the metal raw material, and the amount of sodiumhydroxide was controlled by a pH controller. Next, the reaction wascarried out at 50° C. for about 25 hours while the reaction mixture inthe reactor was stirred, and then the introduction of the metalprecursor mixture was stopped.

A slurry solution in the reactor was filtered and washed withhigh-purity distilled water, and then, dried in a hot-air oven at 150°C. for 24 hours, thereby obtaining a metal hydroxide precursor(Ni_(0.6)Co_(0.2)Mn_(0.2)(OH)₂) powder.

The active material prepared in Comparative Example 2 was evenly alignedfrom an outside to the center of the active material, and a size of theactive material was large, which did not facilitate migration of lithiumions.

MANUFACTURE EXAMPLE 1 Preparation of Coin Cell

The secondary particle of the nickel-based active material(LiNi_(0.6)Co_(0.2)Mn_(0.2)O₂) of Example 1 was used as the positiveactive material for a coin cell as follows.

96 g of the secondary particle of the nickel-based active material(LiNi_(0.6)Co_(0.2)Mn_(0.2)O₂) of Example 1, 2 g of polyvinylidenefluoride, 15 g of N-methylpyrrolidone, as a solvent, and 2 g of carbonblack, as a conductive agent, were mixed. Bubbles were removed from themixture by using a mixer, thereby preparing a homogeneously dispersedslurry for forming a positive active material layer.

The slurry obtained therefrom was coated on an aluminum film by using adoctor blade to form a thin electrode plate. The thin electrode platewas then dried at a temperature of 120° C. for 3 hours or more, andthen, pressed and vacuum-dried to form a positive electrode.

The positive electrode and lithium metal, as a counter electrode, wereused to manufacture a 2032-type (or kind) of coin cell. A separator(thickness: about 16 μm) formed of a porous polyethylene (PE) film wasdisposed between the positive electrode and the lithium metal counterelectrode. Then, an electrolyte was provided thereto, thereby completingmanufacturing a 2032-type (or kind) of coin cell. As the electrolyte, asolution containing 1.15 M LiPF₆ dissolved in a mixed solvent ofethylene carbonate (EC), ethylmethylcarbonate (EMC), anddimethylcarbonate (DMC) at a volumetric ratio of 2:4:4 was used.

MANUFACTURE EXAMPLE 2 Preparation of Coin Cell

A coin cell was manufactured in substantially the same manner as inManufacture Example 1, except that the secondary particle of thenickel-based active material of Example 2 was used instead of thesecondary particle of the nickel-based active material of Example 1.

MANUFACTURE EXAMPLE 3 Preparation of Full Cell

A 18650 cylindrical full cell was prepared in substantially the samemanner as in Manufacture Example 1, except that a negative electrodeprepared in the following manner instead of the lithium metal counterelectrode was used as a negative electrode.

Natural graphite, carboxymethylcellulose (CMC), and styrene butadienerubber (SBR) were mixed to prepare a composition for forming a negativeactive material layer, and the composition was coated and dried on acopper current collector to prepare a negative electrode. A weight ratioof the natural graphite, CMC, and SBR was 97.5:1:1.5, and an amount ofdistilled water was about 50 parts by weight based on 100 parts byweight of the total weight of the natural graphite, CMC, and SBR.

COMPARATIVE MANUFACTURE EXAMPLES 1 and 2 Preparation of Coin Cell

Coin cells were each prepared in substantially the same manner as inManufacture Example 1, except that the nickel-based active materialsprepared in Comparative Examples 1 and 2 were used, respectively,instead of the nickel-based active material prepared in Example 1.

COMPARATIVE MANUFACTURE EXAMPLES 3 and 4 Preparation of Full Cell

Full cells were prepared in substantially the same manner as inComparative Manufacture Examples 1 and 2, except that a negativeelectrode prepared in the following manner instead of the lithium metalcounter electrode was used as a negative electrode.

Natural graphite, CMC, distilled water, and SBR were mixed to prepare acomposition for forming a negative active material layer, and thecomposition was coated and dried on a copper current collector toprepare a negative electrode. A weight ratio of the natural graphite,CMC, and SBR was 97.5:1:1.5, and an amount of distilled water was about50 parts by weight based on 100 parts by weight of the total weight ofthe natural graphite, CMC, and SBR.

EVALUATION EXAMPLE 1 Analysis using Scanning Electron Microscope (SEM)

Secondary particles of the nickel-based active materials of Example 1and Comparative Examples 1 and 2 were analyzed by using a scanningelectron microscope (SEM). The SEM used herein was a Magellan 400 L(manufactured by FEI Company). A pretreatment was performed on a samplecross-section by milling using a CP2 (manufactured by JEOL) for 4 hoursat a voltage of 6 kV and a current of 150 uA. Then, the SEM analysis wasperformed under the conditions of 350 V.

The results of analyzing the nickel-based active materials prepared inExample 1 and Comparative Examples 1 and 2 by using an SEM are shown inFIGS. 3A-3C, respectively.

Referring to FIGS. 3A-3C, the nickel-based active material secondaryparticles prepared in Example 1 were found to have at least two radialcenters. The primary particle structures were found to have a porousinner portion and an outer portion including an radial arrangement andaligned in an isotropic arrangement, thereby forming a nickel-basedactive material secondary particle. In addition, it was found that theprimary particles are aligned in an isotropic arrangement, therebyforming a primary particle structure. In contrast, the nickel-basedactive materials of Comparative Example 1 and Comparative Example 2 werefound not to have two or more radial centers.

EVALUATION EXAMPLE 2 SEM and Sphericity

Secondary particles of the nickel-based active materials of Example 1and Comparative Examples 1 and 2 were analyzed by using an SEM. The SEMused herein was a Magellan 400 L (manufactured by FEI Company). Apretreatment was performed on a sample cross-section by milling using aCP2 (manufactured by JEOL) for 4 hours at a voltage of 6 kV and acurrent of 150 uA. The results of the analysis performed on thesecondary particles of the nickel-based active materials by using a SEMare shown in FIGS. 3A-3C, respectively.

Referring to FIGS. 3A-3C, it was found that particles of thenickel-based active materials of Comparative Examples 1 and 2 have aspherical shape, whereas the secondary particles of the nickel-basedactive material of Example 1 have a non-spherical shape, which includeda structure including at least two radial centers.

Also, a sphericity of the most frequent particle in the secondaryparticle of the nickel-based active material of Example 1 was measured.The sphericity was measured by using an FPIA-3000 (manufactured bySysmex Corporation, Japan). In the measurement by using FPIA-3000,samples for the measurement were each prepared by adding a surfactant to50 to 100 ml of distilled water, and adding 10 to 20 mg of the secondaryparticle of the nickel-based active material of Example 1 thereto,sonicating the resultant in a sonicator for 1 minute. The sphericity isautomatically obtained by the FPIA-3000 according to Equation 1.

Sphericity={2×(area×π)^(1/2)}/(perimeter)   Equation 1

In Equation 1, the area refers to an area of a projected particle, andthe perimeter refers to a circumferential length of a circle having thesame area with that of the projected particle. A value of the sphericitymay be in a range of about 0 to about 1.

The closer the value is 1, the more circular the particle is, and thecloser the value is 0, the more linear the particle is.

TABLE 1 Sample Sphericity Example 1 0.970 Example 2 0.980 ComparativeExample 1 0.995 Comparative Example 2 0.990

Referring to Table 1, sphericities of the secondary particles of thenickel-based active materials prepared in Examples 1 and 2 were smallerthan those of the secondary particles of nickel-based active materialsprepared in Comparative Examples 1 and 2.

EVALUATION EXAMPLE 3 Measurement of Porosity and Pore Size in InnerPortion of Active Material

The secondary particles of the nickel-based active material of Example 1were analyzed by using an SEM. The SEM used herein was a Magellan 400 L(manufactured by FEI Company). A pretreatment was performed on a samplecross-section by milling using a CP2 (manufactured by JEOL) for 4 hoursat a voltage of 6 kV and a current of 150 uA. An SEM analysis of asurface and a cross-section of the nickel-based active materialsecondary particle was performed. Referring to the results, primaryparticles of the nickel-based active material were plate particles mostof which have a plate shape aligned in a radial arrangement, but someprimary particles of the nickel-based active material are arranged in anon-radial arrangement. Here, an amount of the non-radial plateparticles was about 3 wt % based on the total weight of the radial plateparticle and the non-radial plate particles. Average lengths, averagethicknesses, and average ratios (average length/average thickness) ofthe plate particles were calculated, and the results are shown in Table2.

TABLE 2 Cross- Surface section Average length (nm) 290 Average length(nm) 360 Average thickness (nm) 130 Average thickness (nm) 150 Averageratio 2.3 Average ratio 2.9

In Table 2, the plate particles denote particles constituting a radiallyarranged structure in the outer portion of the active material.

EVALUATION EXAMPLE 4 Charging/Discharging Characteristics

1) Manufacture Examples 1 and 2 and Comparative Manufacture Examples 1and 2

The coin cells of Manufacture Examples 1 and 2 and ComparativeManufacture Examples 1 and 2 were charged and discharged once at 0.1 Cto perform a formation process. Then, the coin cells were charged anddischarged once at 0.2 C to identify initial charging and dischargingcharacteristics. As the coin cells were repeatedly charged anddischarged at 1 C for 50 times at 45° C., the charging and dischargingcharacteristics were observed. During charging, the charging began in aconstant current (CC) mode, and then, the mode was changed into aconstant voltage (CV) mode which is set to cut off at 0.05 C and 4.3 V;and during discharging, the cut-off was set at 3.0 V in a CC mode.

The charging/discharging characteristics are shown in Table 3. Also,voltage changes according to capacities of the coin cells of ManufactureExample 1 and Comparative Manufacture Examples 1 and 2 are shown in FIG.4.

TABLE 3 Charging/discharging Sample efficiency (%) Manufacture Example 195.1 Manufacture Example 2 93.5 Comparative Manufacture Example 1 90.9Comparative Manufacture Example 2 93.3

Referring to Table 3 and FIG. 4, lithium diffusion resistance of thecoin cells prepared in Manufacture Examples 1 and 2 significantlydecreased within the initial charging voltage range, and thuscharging/discharging efficiencies of the lithium secondary batteries atsubstantially the same charge capacity (using an active material havingthe same transition metal composition) improved, compared to those ofComparative Manufacture Examples 1 and 2.

2) Manufacture Example 3 and Comparative Manufacture Examples 3 and 4

Charging/discharging characteristics of the full cells prepared inManufacture Example 3 and Comparative Manufacture Examples 3 and 4 wereevaluated in the same manner as in the method used to evaluatecharging/discharging characteristics of the coin cells prepared inManufacture Examples 1 and 2 and Comparative Manufacture Examples 1 and2.

The results of evaluation of the charging/discharging characteristicsare shown in FIGS. 5-6.

Referring to FIGS. 5-6, the full cell prepared in Manufacture Example 3had improved lifespan characteristics compared to those of the fullcells prepared in Comparative Manufacture Examples 3 and 4.

EVALUATION EXAMPLE 5 Scanning Electron Microscope

SEM analysis was performed on cross-sections of the positive electrodesprepared in Manufacture Example 1 and Comparative Manufacture Example 1.The SEM used herein was a Magellan 400 L (manufactured by FEI Company).

Referring to the results of the SEM analysis of the positive electrodesprepared in Manufacture Example 1 and Comparative Manufacture Example 1,it was confirmed that, unlike the positive electrode of ComparativeManufacture Example 1, the positive electrode of Manufacture Example 1contained a positive active material having a structure in which radialcenters were empty. When the positive electrode of Manufacture Example 1is used, stress on the positive active material in the positiveelectrode is suppressed or reduced due to a buffer effect on changes involume during a charge/discharge process.

As described above, according to one or more embodiments, a nickel-basedactive material for a lithium secondary battery may have a decreasedlithium diffusion resistance. When a lithium secondary battery includesa positive electrode containing the nickel-based active material, thelithium secondary battery may have improved discharge capacity andcharging/discharging efficiency and long lifespan characteristics.

It should be understood that embodiments described herein should beconsidered in a descriptive sense only and not for purposes oflimitation. Descriptions of features or aspects within each embodimentshould typically be considered as available for other similar featuresor aspects in other embodiments.

Spatially relative terms, such as “beneath,” “below,” “lower,” “under,”“above,” “upper,” and the like, may be used herein for ease ofexplanation to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. It will beunderstood that the spatially relative terms are intended to encompassdifferent orientations of the device in use or in operation, in additionto the orientation depicted in the figures. For example, if the devicein the figures is turned over, elements described as “below” or“beneath” or “under” other elements or features would then be oriented“above” the other elements or features. Thus, the example terms “below”and “under” can encompass both an orientation of above and below. Thedevice may be otherwise oriented (e.g., rotated 90 degrees or at otherorientations) and the spatially relative descriptors used herein shouldbe interpreted accordingly.

It will be understood that when an element or layer is referred to asbeing “on,” “connected to,” or “coupled to” another element or layer, itcan be directly on, connected to, or coupled to the other element orlayer, or one or more intervening elements or layers may be present. Inaddition, it will also be understood that when an element or layer isreferred to as being “between” two elements or layers, it can be theonly element or layer between the two elements or layers, or one or moreintervening elements or layers may also be present.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the presentdisclosure. As used herein, the singular forms “a” and “an” are intendedto include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises,” “comprising,” “includes,” and “including,” when used inthis specification, specify the presence of the stated features,integers, acts, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,integers, acts, operations, elements, components, and/or groups thereof.

As used herein, the terms “substantially,” “about,” and similar termsare used as terms of approximation and not as terms of degree, and areintended to account for the inherent deviations in measured orcalculated values that would be recognized by those of ordinary skill inthe art. Further, the use of “may” when describing embodiments of thepresent disclosure refers to “one or more embodiments of the presentdisclosure.” As used herein, the terms “use,” “using,” and “used” may beconsidered synonymous with the terms “utilize,” “utilizing,” and“utilized,” respectively. Also, the term “exemplary” is intended torefer to an example or illustration.

Also, any numerical range recited herein is intended to include allsub-ranges of the same numerical precision subsumed within the recitedrange. For example, a range of “1.0 to 10.0” is intended to include allsubranges between (and including) the recited minimum value of 1.0 andthe recited maximum value of 10.0, that is, having a minimum value equalto or greater than 1.0 and a maximum value equal to or less than 10.0,such as, for example, 2.4 to 7.6. Any maximum numerical limitationrecited herein is intended to include all lower numerical limitationssubsumed therein, and any minimum numerical limitation recited in thisspecification is intended to include all higher numerical limitationssubsumed therein. Accordingly, Applicant reserves the right to amendthis specification, including the claims, to expressly recite anysub-range subsumed within the ranges expressly recited herein.

While one or more embodiments have been described with reference to thefigures, it will be understood by those of ordinary skill in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope as defined by the following claims,and equivalents thereof.

What is claimed is:
 1. A positive electrode comprising a nickel-basedactive material for a lithium secondary battery, the nickel-based activematerial comprising: at least one secondary particle comprising at leasttwo primary particle structures, wherein the at least two primaryparticle structures each comprise a porous inner portion and an outerportion comprising a radially arranged structure, wherein the at leastone secondary particle comprises at least two radial centers, whereinthe at least two primary particle structures each comprise anagglomeration of at least two primary particles, and wherein respectiveones of the at least two primary particles of each primary particlestructure are radially arranged around a respective one of the at leasttwo radial centers.
 2. The positive electrode of claim 1, wherein the atleast two primary particle structures are aligned in a multicenterisotropic arrangement to form the secondary particle.
 3. The positiveelectrode of claim 1, wherein the at least two primary particlestructures have a plurality of particle shapes, and wherein a sphericityof the most abundant particle shape of the at least two primary particlestructures have is in a range of about 0.960 to about 0.985.
 4. Thepositive electrode of claim 1, wherein a size of each of the at leasttwo primary particle structures is in a range of about 2 μm to about 5μm.
 5. The positive electrode of claim 1, wherein a size of the at leastone secondary particle is in a range of about 5 μm to about 25 μm. 6.The positive electrode of claim 1, wherein a pore size of the porousinner portion is in a range of about 150 nm to about 1 μm, a porosity ofthe porous inner portion is in a range of about 5% to about 15%, and aporosity of the outer portion having a radially arranged structure is ina range of about 1% to about 5%.
 7. The positive electrode of claim 1,further comprising a plate particle of which a long axis is aligned in aradial direction, and a ratio of a thickness to a length of the plateparticle is in a range of about 1:2 to about 1:20.
 8. The positiveelectrode of claim 1, wherein the nickel-based active material comprisesa compound represented by Formula 1:Li_(a)(Ni_(1-x-y-z)CO_(x)Mn_(y)Mz)O₂   Formula 1 wherein, in Formula 1,M is an element selected from the group consisting of boron (B),magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), titanium(Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zirconium(Zr), and aluminum (Al); and a, x, y, and z satisfy the followingconditions: 0.95≤a≤1.3x≤(1-x-y-z), y≤(1-x-y-z), 0<x<1, 0≤y<1, 0≤z<1, and0<(1-x-y-z)<1.
 9. The positive electrode of claim 8, wherein an amountof nickel in the nickel-based active material is in a range of about 30mol % to about 95 mol % based on a total amount of transition metalsincluding nickel, cobalt, and manganese, the amount of nickel beinghigher than an amount of each of manganese and cobalt.
 10. The positiveelectrode of claim 1, wherein the nickel-based active material comprisesLiNi_(0.6)Co_(0.2)Mn_(0.2)O₂, LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂,LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂, and/orLiNi_(0.85)Co_(0.1)Al_(0.05)O₂.
 11. A nickel-based active material for alithium secondary battery, the nickel-based active material comprising:at least one secondary particle comprising at least two primary particlestructures, wherein the at least two primary particle structures eachcomprise a porous inner portion and an outer portion comprising aradially arranged structure, wherein the at least one secondary particlecomprises at least two radial centers, wherein the at least two primaryparticle structures each comprise an agglomeration of at least twoprimary particles, and wherein a porosity in the porous inner portion isin a range of about 5% to about 15%, and a porosity in the outer portioncomprising the radially arranged structure is in a range of about 1% toabout 5%.
 12. The nickel-based active material according to claim 11,wherein the primary particle structure (11) is aligned in a multicenterisotropic arrangement to form the secondary particle (12).
 13. Thenickel-based active material according to claim 11, wherein a sphericityof a most frequent particle of the nickel-based active material is in arange of about 0.960 to about 0.985.
 14. The nickel-based activematerial according to claim 11, wherein a size of the primary particlestructure is in a range of about 2 μm to about 5 μm.
 15. Thenickel-based active material according to claim 11, wherein a size ofthe secondary particle is in a range of about 5 μm to about 25 μm. 16.The nickel-based active material according to claim 11, wherein a poresize in the porous inner portion, is in a range of about 150 nm to about1 μm, and a pore size in the outer portion is in less than about 150 nm.17. The nickel-based active material according to claim 11, furthercomprising a plate particle of which a long axis is aligned in a radialdirection, a ratio of a thickness to a length of the plate particle isin a range of about 1:2 to about 1:20, an average thickness of the plateparticles constituting the outer portion is in a range of about 100 nmto about 250 nm, and an average length of a long axis of the plateparticles is in a range of about 250 nm to about 1100 nm.
 18. Thenickel-based active material according to claim 11, wherein thenickel-based active material is a compound represented by Formula 1:Li_(a)(Ni_(1-x-y-z)CO_(x)Mn_(y)M_(z))O₂   Formula 1 wherein, in Formula1, M is an element selected from the group consisting of boron (B),magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), titanium(Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zirconium(Zr), and aluminium (Al); and a, x, y, and z satisfy the followingconditions: 0.95≤a≤1.3, x≤(1-x-y-z), y≤(1-x-y-z), z≤(1x-y-z), 0<x<1,0≤y<1, 0≤z<1, and 0<(1-x-y-z)<1.
 19. The nickel-based active materialaccording to claim 18, wherein an amount of nickel in the nickel-basedactive material is in a range of about 30 mol % to about 95 mol % basedon a total amount of transition metals including nickel, cobalt, andmanganese, the amount of nickel being higher than an amount of each ofmanganese and cobalt.
 20. The nickel-based active material according toclaim 18, wherein the nickel-based active material isLiNi_(0.6)Co_(0.2)Mn_(0.2)O₂, LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂,LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂, orLiNi_(0.85)Co_(0.1)Al_(0.05)O₂.
 21. The nickel-based active materialaccording to claim 18, wherein a closed pore is present in the porousinner portion.
 22. A positive electrode comprising the nickel-basedactive material according to claim
 11. 23. A lithium secondary batterycomprising: a positive electrode comprising the nickel-based activematerial according to claim 11; a negative electrode; and an electrolytebetween the positive electrode and the negative electrode.